In today's radio communications networks a number of different technologies are used, such as Long Term Evolution (LTE), LTE-Advanced, 3rd Generation Partnership Project (3GPP) Wideband Code Division Multiple Access (WCDMA) system, Global System for Mobile communications/Enhanced Data rate for GSM Evolution (GSM/EDGE), Worldwide Interoperability for Microwave Access (WiMax), or Ultra Mobile Broadband (UMB), just to mention a few. The radio communications networks of today are moving towards more advanced multi antenna solutions to improve capacity and coverage. Multi antenna solutions, such as Multiple Input Multiple Output (MIMO), are one of the corner-stones in for example the LTE standard. LTE Rel 8 support up to 4 different transmit antenna ports, supporting both beam forming and diversity schemes for better coverage and multi stream for better capacity. LTE is a Frequency Division Multiplexing technology wherein Orthogonal Frequency Division Multiplexing (OFDM) is used in a downlink (DL) transmission from a radio base station to a user equipment. Single Carrier-Frequency Domain Multiple Access (SC-FDMA) is used in an uplink (UL) from the user equipment to the radio base station.
The system information in LTE is transmitted from a radio base station over a cell with a transmission (TX)-diversity scheme using all available antenna ports to improve the robustness of the signaling. To determine the number of antenna ports active in the cell a user equipment (UE) must first do a blind decoding of the first system information block, the Master Information Block (MIB). I.e. the UE does a hypothesis test that the cell is configured with a number of antenna ports, 1, 2 or 4, and if the UE manages to decode the MIB it assumes that it guessed correctly. This is a cost efficient way to convey the number of antenna-ports used that enables full utilization of the ports, but it also makes dynamic changes of the number of antenna ports hard to do, since the LTE standard does not say anything on when, if ever, the UE can be expected to re-do the hypothesis test.
To support demodulation and channel estimation each antenna port carries reference symbols or a set of reference symbols. The reference symbols are used for demodulation reference at data transmissions, channel estimation that the UE feeds back to the radio base station, handover measurements and/or for cell search. Since the radio base station cannot know when the UE may do cell search and to some extent handover measurements the reference symbols are transmitted at all time over the cell.
According to current technologies, a packet data shared channel (PDSCH) is to be transmitted on a same set of antenna ports as a physical broadcast channel (PBCH), which is one of the set of antenna ports {0}, {0, 1} or {0, 1, 2, 3}, in resource blocks in which UE-specific reference symbols are not transmitted.
Since the PBCH must be readable by all UEs, being in active mode and idle mode, that are located in the cell area this clearly states that it is not allowed to do beam-forming, as PDSCH shall use the same antenna ports as PBCH, on the individual antenna elements in LTE Rel-9 unless UE specific reference symbols are transmitted. Beam-forming is a signal processing technique used for direct signal transmission or reception towards a position or area.
There are two drawbacks of using UE specific reference symbols; reduced transmission rank and loss of resource elements.
First, the UE specific reference symbols in Rel-8 of LTE are limited to one antenna port, that is antenna port {5} in Rel-8, while the cell specific reference symbols are defined for up to 4 antenna ports {0, 1, 2, 3}. In Rel-9 there are two additional antenna ports {7, 8} for which UE specific reference symbols are defined that support spatial multiplexing of up to rank 2. Spatial multiplexing is defined as to transmit independent and separately encoded data signals, so-called streams, from each of the multiple transmit antennas. Therefore, a space dimension is reused, or multiplexed, more than one time.
Thus, for Rel-8 and Rel-9 only transmission rank 1, and 2 respectively, are possible when using UE specific reference symbols while rank 4 is supported when using the cell specific reference symbols as demodulation reference for the PDSCH. The “transmission mode” of the UE determines which antenna port to use. For example, in single layer transmission mode 7 antenna port 5 is used for transmission and single or dual layer transmission mode 8 antenna port 7 and/or antenna port 8 may be used for transmission.
Second, the UE specific reference symbols consume resource elements that could otherwise be used by data symbols. Antenna port 5, used for a transmission with UE specific reference symbols using only a single layer or stream, consumes 12 resource elements and antenna ports 7 and 8 also consume 12 resource elements in total. In a cell where all 4 cell specific antenna ports {0, 1, 2, 3} are used there are already 24 resource elements used and with UE specific reference symbols another 12 references symbols for demodulation reference is used.
Thus, transmissions of higher rank must today use the same antenna ports as the PBCH which are transmitted all over the cell of the radio base station and requires a lot of radio resources. The other antenna ports use UE specific reference symbols which enable only up to rank 2 and consume radio resources as well. Thus, to obtain a good performance of communications within the radio communications network a lot of radio resources are required or else one cannot use higher transmission ranks.